1. Field of the Invention
The present invention relates to fixing apparatuses used in image forming apparatuses and to image forming apparatuses provided with these fixing apparatuses.
2. Description of Related Art
In electrophotographic image forming apparatuses that are commonly employed in copiers, laser printers, and facsimile machines and the like, it has long been common for thermal fixing systems to be used as a fixing system used in fixing apparatuses, and hot roller fixing systems using a hot roller are ordinarily employed as the thermal fixing system. In these hot roller fixing systems, a hot roller that is internally provided with a heater of a heat source and whose outer circumference is covered with a rubber or a resin having excellent releasability and a pressure roller are caused to press against each other such that a transfer paper on which a toner image has been formed passes through a nip portion formed between these rollers, thereby thermally melting the toner, and fixing is carried out by causing the toner to fuse onto the transfer paper. These hot roller fixing systems are applied in high speed operations since the entire hot roller is maintained at a predetermined temperature.
However, in recent years, full color image forming apparatuses such laser printers and the like that support full color recording have become widely used, and toners of four colors of magenta, yellow, cyan, and black are used in these full color image forming apparatuses. Unlike a case of fixing a single color toner in which the toner is fixed by simply softening it while pressure is applied, to fix a full color toner image in a full color image forming apparatus, it is necessary to mix multiple types of color toners in a near-melted state, and therefore it is necessary for the fixing apparatus to put the toner into a completely melted state.
Thus, in the fixing apparatuses of hot roller fixing systems in full color image forming apparatuses, an elastic structure of a rubber layer formed by a silicone rubber or the like is provided on a support structure of a metal or the like having superior thermal conductivity, then a surface of the elastic structure is covered by a fluorocarbon resin having excellent releasability, thereby forming a hot roller.
However, even in fixing apparatuses of a hot roller fixing system using a hot roller such as this, when commencing operation of the image forming apparatus, it is necessary to heat the hot roller having a rubber layer of poor thermal conductivity using a heat source such as a heater provided inside the hot roller until the hot roller reaches a predetermined temperature. For this reason, time is required from powering up until operation is possible, which is a problem in that a waiting time is produced. Furthermore, there is a problem that the temperature of the hot roller drops during high speed continuous operations.
Accordingly, to address these problems in recent years, fixing apparatuses of a fixing system (belt-type external heating fixing system) have been proposed (see JP 2005-292714A and JP 2007-241180A for example) in which a heating belt of a belt-type external heating means provided with a heating belt, which rotates while being heated, is brought in contact with a surface of the hot roller, such that the hot roller is heated not only from the inside, but from the outside as well.
The invention described in JP 2005-292714A relates to a fixing apparatus of the above-mentioned belt-type external heating fixing system, in which in order to increase the amount of heat conducted to the hot roller from the belt-type external heating means, at least one heat source provided in the belt-type external heating means is arranged inside a contact region between the hot roller and the heating belt, and temperature drops of the heating belt are avoided by directly heating the heating belt using the heat source, such that the amount of heat conducted to the hot roller is increased and the temperature of the heat source is controlled using a temperature detection member.
The invention described in JP 2007-241180A relates to a fixing apparatus of the above-mentioned belt-type external heating fixing system, in which the amount of heat conducted to the hot roller is increased by causing the heating belt of a belt-type external heating means, which is provided with a heating belt, which is heated while rotating, to contact a surface of the hot roller, and the amount of conducted heat is controlled using a means capable of varying a contact surface area of the heating belt to the hot roller.
As described above, a fixing apparatus of the above-mentioned belt-type external heating fixing system is configured as shown in FIG. 7 for example. That is, the fixing apparatus of the belt-type external heating fixing system in FIG. 7 fixes unfixed images onto the recording material by causing the recording material to pass through a nip portion formed by contact between a rotating hot roller 90 and an unshown pressure roller.
In the fixing apparatus of this belt-type external heating fixing system, a heating belt 91 contacts and rotates against the hot roller 90. The heating belt 91 is suspended between and supported by support rollers 92 and 93, which are internally provided with heat sources 94 and 95. The support rollers 92 and 93 are heated by the heat sources 94 and 95, and heat is conducted from the thus-heated support rollers 92 and 93 to the heating belt 91.
The heat that is conducted to the heating belt 91 is further conducted from the heating belt 91 to the hot roller 90. Furthermore, in this fixing apparatus of the belt-type external heating fixing system, the support roller 92 and the support roller 93 contact the hot roller 90 through the heating belt 91. Furthermore, in this fixing apparatus of the belt-type external heating fixing system, in contrast to the internal heat sources provided inside the hot roller 90 for heating the hot roller 90, a portion constituted by the support rollers 92 and 93, the heat sources 94 and 95 provided inside the support rollers 92 and 93, and the heating belt 91 is referred to as an external heating member. It should be noted that the arrow shown in FIG. 7 indicates the rotation direction of the heating belt 91.
In this regard, the fixing apparatus of the belt-type external heating fixing system shown in FIG. 7 has problems such as the following. That is, due to thermal expansion of end portions of the support rollers 92 and 93, or thermal expansion or the like of shaft bearing portions, one of the support roller 92 and the support roller 93 may sometimes not rotate while the heating belt 91 is rotating. Specifically, there is a state such as the following when for example the support roller 92 is rotating and the support roller 93 is not rotating.
In FIG. 7, the support rollers 92 and 93 receive heat from the heat sources 94 and 95, and convey heat to the heating belt 91, and the continually rotating support roller 92 is capable of conveying heat from its entire surface to the heating belt 91. However, the support roller 93, which is not rotating, supplies heat only at an A region (FIG. 7) that contacts the heating belt 91, and a B region (FIG. 7) accumulates heat.
Since the support rollers 92 and 93 normally rotate together, heat that has accumulated in the heat sources 94 and 95 and heat that has accumulated in the support rollers 92 and 93 can be diffused by a post-rotation operation carried out in continuation from completion of the temperature raising operation after powering up or completion of a fixing operation. In this way, the external heating member can be prevented from rising in temperature abnormally after a rotation stoppage of the hot roller 90.
However, in a case where the support roller 93 is not rotating, since the support roller 93 is stopped in a state in which heat accumulated in the B region (FIG. 7) is not diffused, the external heating member rises in temperature abnormally. In this case, after the rotation stoppage of the support roller 93, the surface temperature of the hot roller 90 at a C portion, which is in contact with the not-rotating support roller 93, is higher compared to the surface temperatures of other portions of the hot roller 90.
The same is also conceivable in a case where the support roller 93 is rotating and the support roller 92 is not rotating, since it is merely that the conditions of the support roller 93 and the support roller 92 are opposite to the above.
Thus, in a case where a next fixing command is issued and a fixing operation is carried out in a state where temperature unevenness has been produced on the surface of the hot roller 90, a situation arises in which a problem is produced of uneven glossiness in the outputted image of the recording material that has undergone fixing. Further still, in a case where the temperature of the external heating member undesirably exceeds a heat-resistance temperature of the surface material of the hot roller 90, a situation arises in which problems are produced such as wrinkling in the surface of the hot roller 90.
Two support rollers, namely the support roller 92 and the support roller 93, are used in the above-described example, but the above-described problems still occur in a case where three or more support rollers are used.